A communication system which utilizes media having a problem with communication quality is provided with a control station for controlling media access of other station(s) in order to utilize media as efficiently as possible. The other station is regarded as a controlled station, i.e., the control station and the controlled station have a master-slave relationship. In this manner, a central control type communication system protocol which can improve the efficiency of the entire communication system has been developed.
When such a protocol is applied to a wireless LAN, a power line communication system and the like, the efficiency of media access in the communication system is significantly influenced by communication quality of a control station and a controlled station which depends on, for example, a physical positional relationship therebetween.
Japanese Patent Laid-Open Publication No. H11-8585 (Patent Publication 1) discloses a terminal which operates in a control station mode and in a controlled station mode (two operational forms) and can select the operational forms. In a communication system described in Patent Publication 1, the terminal detects the operating modes of surrounding terminals after being powered on, and when there is no detected terminal which is activated as a control station, operates as a control station.
In a communication system having a control station, it is necessary to consider the possibility that the control station gets out of order during operation. Further, in a power line communication system or the like, it is considered that a station is repeatedly connected and disconnected. When a control station is disconnected from the communication system, it is necessary to, for example, reconstruct a control station in order to achieve efficient media access. However, in the technology of Patent Publication 1, it is not possible to reconstruct a control station, so that the communication quality of a communication system cannot be maintained.
Japanese Patent No. 2563935 (Patent Publication 2) proposes a concept that a control station transfers its information to a controlled station (another station) during operation, and when the control station is suddenly disconnected from a network, the controlled station quickly replaces the control station to maintain network quality (Patent Publication 2).
However, in a network employing a medium, such as a radio wave or a power line, in which the reliability of transmission characteristics varies significantly depending on a physical positional relationship between stations, the location of a control station has a significant influence on the transmission efficiency of the whole network. Therefore, unless a control station is determined, taking into consideration the location on a network of connection of a terminal, the performance of a terminal, or the like, there is a possibility that the communication quality is significantly impaired upon replacement.
In a network in which importance is particularly placed on the transmission efficiency and the Quality of Service (QoS) guarantee required by an application, a procedure for changing control stations needs to be sufficiently prepared in advance. Furthermore, a control station may be accidentally disconnected before giving information as described in Patent Publication 2. In this case, it may be difficult to reconstruct a control station using latest information, a control request which has been once accepted may be lost so that latest information cannot be inherited, or a network system itself may not function.
A terminal which operates as a control station is often designed to be connected to a location suitable for control of a network, taking the importance of the function thereof into consideration. Also, it is often that a control station is capable of recovering itself quickly if it is disconnected from a network for some reason. Therefore, when a control station is disconnected from a communication system, in general it is not always satisfactory to replace the control station with a new one. It is often satisfactory that the original control station returns to a media access control.